Volume range expansion system



Jan. 31, 1939. LJBEERS 2, 3-

VOLUME RANGE EXPANSION SYSTEM Filed March El, 1956 4- Qkg 3nventor Patented Jan. 31, 1939 UNlTED STATES PATENT OFFIQE George L. Beers, Collingswood, N. .l., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1936, Serial No. 71,942

3 Claims.

The present invention relates to volume range expansion systems, and has for its primary object to provide an improved control system of that character wherein volume range expansion is effected in a superaudible or intermediate frequency amplifier preceding an audio frequency detector thereby to prevent distortion.

It is also an object of the present invention to provide a volume range expansion system which operates to control the gain of a modulated carrier wave amplifier with the rectified modulation signal on the carrier wave which is separately amplified.

By applying gain control to a high frequency or modulated carrier wave amplifier preceding the detector and audio frequency amplifier it has been found that the distortion so often encountered When volume range expansion is accomplished in an audio frequency amplifier may be minimized.

A system in accordance With the invention is particularly adapted for use in connection with radio receiving systems, wherein many volume expansion arrangements involving variation of the gain of audio frequency amplifiers has been found to be impractical. Generally, volume expansion systems operating in conjunction with audio frequency amplifiers of radio receivers tend to introduce distortion if the signal voltage on the expander amplifier is not kept relatively low.

In applying the system of the present invention to a high frequency amplifier stage or stages, preceding an audio frequency or modulation detector, it is possible to avoid distortion of the signal even at relatively high signal amplitude. Accordingly, it is a further object of the present invention to provide an improved volume expansion system which is not limited to operation at low signal amplitudes.

In applying the invention to a practical control system a proper controlling potential such as the rectified modulation component of the received modulated signal is applied to a super audible frequency or intermediate frequency amplifier to control the sensitivity or the gain therein, the modulation or audio frequency component of the rectified signal being derived through a separate high frequency amplifier channel and rectifier connected substantially in parallel with the main signal channel through the modulation or audio frequency detector.

It has been found that volume range expansion for phonograph record reproduction may be provided by modulating an oscillator with the phonograph signals and controlling the gain of an amplifier for the modulated oscillations in accordance with the variations in modulation, as will hereinafter be described in connection with a preferred embodiment of the invention.

The invention will be more clearly understood 5 from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing the figure is a schematic circuit diagram of a volume range expansion system embodying the invention.

Referring to the drawing, 5, 6 and l are electric discharge amplifier devices or tubes forming a multi-stage superaudible frequency amplifier be- 1; tween modulated signal input terminals 8 and an audio frequency or modulation detector 9. This amplifier may be an intermediate frequency amplifier for a superheterodyne receiver or the like, and the detector 9' may be the main signal or .20 second detector coupled to a first stage audio frequency amplifier l6 and an output amplifier ii for supplying the audio frequency or modulation signal to an output device such as a loudspeaker l2. The amplifier 5, G, l and rectifier 9 5 together with amplifiers Ill and H represent any suitable modulated carrier wave amplifying channel for modulated oscillations.

The first and second stage intermediate frequency amplifiers 5 and 6 are provided with auto- .30 matic volume control means 43 of any suitable type for maintaining the gain through said stages in accordance with the signal strength to provide the usual automatic control of volume. The intermediate frequency amplifier stage represented by the tube l is separately controlled by means including a diode rectifier M, which is connected through an amplifying channel l5, separate from and parallel with, the main signal channel, being connected therewith at a point H5 at the same input circuit ll as the stage 1 which precedes the detector 9 and follows the automatic volume controlled stages 5 and *3.

The separate channel for the automatic volume control means id comprises an intermediate fre quency amplifier device l8, an auxiliary detector and amplifier device l9 corresponding to the combined detector amplifier ii, an audio or modulation frequency amplifier 2a which is coupled to the diode Hi to provide a biasing potential variable in accordance with audio frequency signal strength or modulation in the audio frequency or modulation output resistor 2!. This biasing potential is applied to the grid circuit ll of the intermediate frequency amplifier l preceding .555

the detector 9, by a connection for said circuit through a bias supply lead 22 with the positive terminal 23 of the output resistor 2|. The negative terminal indicated at 24 is connected through a supply lead 25 with the negative terminal 26 of means 21 providing a source of potential when the anode supply terminals 28 are energized. The positive terminal 29 of the potential supply means 2'! is connected with the cathode 30 of the amplifier device 1, whereby the polarities of the variable source 2| and the fixed source 2! are in opposition. The potential supplied by the resistor 2'! is normally such that with no signal being received, the separate amplifying channel and the diode l4 provide no biasing potential at 2| and, accordingly, the intermediate frequency amplifier stage I is arranged to amplify with a predetermined gain, while by action of the automatic volume control means l3- the gain of the stages and 6 is relatively high. As soon as a carrier wave is received, the gain of the first and second intermediate frequency amplifier stages 5 and 6 is reduced by signal rectification, the drop in potential through the diode output resistor 35 being applied to the tubes 5 and B, in addition to the fixed potential provided by the self-bias resistors 35, as is obvious in the system disclosed. In the intermediate frequency amplifier stage, preceding the detector and following the automatic volume control stages, the control is substantially the opposite, the gain being increased with an increase in the average amplitude of the modulation or audio frequency component by reason of the amplified signal being rectified and applied through the resistor 2| in opposition to the potential in the resistor 21, to counteract the potential supplied by the latter, and thereby to reduce the negative bias on the amplifier device 1, whereby the gain is correspondingly increased, and a desired degree of volume expansion may be obtained. In order that the increase in gain in accordance with audio frequency component of the signal may be independent of the operation of the signal channel, the separate amplifying channel I5 is provided with a detector 9 and audio frequency amplifier 20 as well as a preceding intermediate frequency amplifier IS. The rectification then takes place at audio frequency in the device it and it is the audio frequency component of the signal which supplies current to be rectified in the control resistor 2|. With the system as above described, the greater the amplitude of the audio frequency signal impressed upon the diode M, the more the gain in the amplifier tube 7 is increased, thereby accelerating the volume range of the reproduced signals from the loud speaker l2. Since a volume expansion of only 5 to times may be desired in certain practical cases, this change in gain can easily be secured by varying the bias of the tube 1 without introducing noticeable distortion.

It should be understood that the tubes illustrated may be of any desired type such as pentodes having exponential characteristics, triodes being shown merely for the sake of simplicity. This system operates particularly well for phonograph reproduction in connection with an oscillator which is modulated by the pickup signals and received as a high frequency signal in the system shown.

For this purpose the high frequency input terminal 8 may be connected to a source of intermediate frequency represented by terminals 38, through suitable switching means 39 to receive radio signals, such as a modulated carrier wave, for example, at a desired intermediate frequency, or may be connected to another source of modulated oscillations 40, to receive modulated oscillations therefrom, as desired. A phonograph pickup device indicated at 4| may be connected to the oscillator 40 to provide modulation. Thus the volume range expansion channel may be utilized for both radio and phonograph signals by operation of the switch.

For this purpose, the switch 39 is provided with a contact 42 which may be connected with a contact 43, as shown, to provide connection with the input terminals 38 for radio signals, and may be connected with a contact 44 for the modulated oscillator 40, a common connection being made through a lead 45 for both the oscillator and the input terminals.

Any suitable modulated oscillator may be utilized at 4B and, accordingly, the detail construction thereof is not shown. It is sufficient that the oscillator may be modulated by signals received from a phonograph pickup device 4| to provide a modulated carrier wave at the frequency of the amplifier 5, 6 and In certain cases, it may be desirable to remove the automatic volume control |3 from operation in connection with the amplifier stages 5 and 6 and, accordingly, a switch 46 is provided between the rectifier l3 and the amplifier or carrier wave channel to which it is connected.

I claim as my invention:

1. In a radio signal receiving system, the combination of an intermediate frequency amplifier, an audio frequency detector, means for controlling the amplitude of signals applied to said detector, comprising an automatic volume control circuit connected with certain of the amplifying tubes in saidamplifier, rectifier means for producing a direct current potential proportional to the average amplitude of the audio frequency component of said signals, means providing a separate intermediate frequency amplifier channel for said last named rectifier connected with the intermediate frequency amplifier to receive signal energy therefrom at a point preceding the detector and following said first named amplifying tubes, and means for applying said direct current potential to certain others of the tubes in the amplifying channel to control the gain therethrough in opposition to the automatic volume control.

2. In a radio receiving system, the combination with a plurality of modulated carrier wave amplifying stages, a detector and an audio frequency amplifier therefor, of automotive volume control means for at least one of the amplifying stages, a modulated carrier wave amplifier, a detector connected with said amplifier, means for applying modulated carrier wave signals to said last named amplifier from one of said amplifying stages preceding the first named detector and following the automatic volume control means, means for amplifying the audio frequency output from the second named detector, means for rectifying said amplified audio frequency output to provide a .direct current biasing potential proportional to the average amplitude of the modulation, and means for applying said biasing potential to an amplifying stage preceding the first named detector and following the automatic volume control means.

3. In a radio receiving system of the superheterodyne type having an intermediate frequency amplifier provided with initial and subsequent amplifying stages, the method of controlling the signal gain of said system which comprises rectifying a portion of the modulated signal at a predetermined point in the initial stages of said intermediate frequency amplifier and controlling the gain of said system preceding said point in accordance with variations in the amplitude of said rectified portion of the modulated signal, rectifying a further portion of 10 the modulated signal in the subsequent stages of the intermediate frequency amplifier to derive the modulation component thereof, rectifying said modulation component to provide a controlling potential responsive to variations in the amplitude of the modulation, and applying said potential to at least one of the subsequent stages of said amplifier to increase the gain thereof in response to an increase in the amplitude of the modulation component.

GEORGE L. BEERS. 

